Research On Low-temperature Sintering Joining For Ag/Cu Nanoparticle Composite Paste

Research on the preparation of Ag/Cu nanaparticles and their low-temperature sintering joining are of great significance for the developments of both Ag/Cu nanoparticles pastes and lead-free electronic packaging.In this study,Ag nanowires,Ag nanoparticles and Cu nanoparticles have been synthesized by liquid chemical reduction method.Low temperature sintering joining of copper substrates has been conducted with Ag/Cu nanoparticles pastes.The microstructure and properties of the joints have been characterized,and the sintering behavior of nanoparticles and joining mechanism have been studied.Ag nanowires were synthesized by polyol method with the solvent and reducing agent of polyol EG,Ag source of AgNO₃ and dispersant of PVP.The average diameter of Ag nnaowires was about 100 nm.Then the Ag nanoparticles were synthesized by liquid phase chemical reduction method using C6H8O6 acid as reducing agent.The particle size increased with the increase of reaction temperature.When the reaction temperature was 20^oC and 80 ^oC,the average particle diameter of the Ag powders were about 45 nm and 120nm,respectively.Cu nanoparticles were prepared by liquid phase chemical reduction in DEG solution using CuSO4·5H2O as Cu source and NaH2PO2·H2O as reducing agent,and the average diameter of nanoparticles was about 106 nm.Low-temperature sintering joining of bare Cu was realized with Ag nanowires paste.The microstructure of the joint revealed the favorable interfacial bonding between the bare Cu base material and the sintered Ag filler layer.But a small number of holes existed in the filler layer.The sintering behavior of the Ag nanowires mainly included the ends connection,the cross connection and the sides connection.The strength of joint prepared with Ag composite paste I containing Ag nanowires and Ag nanoparticles was higher than that with Ag nanowire paste.This was attributed to the filling of Ag nanoparticles into the pore region formed by the Ag nanowires,thereby leading to the densitification of the filler layer.Low-temperature sintering joining of bare Cu and Cu coated with Ni/Ag films were performed with Ag composite paste II composed of Ag nanoparticles with size of 45 nm and 120 nm.The joint shear strength was 47 MPa and 60 MPa,respectively,suggesting that the Ni/Ag coating contributed to the improvement of joint strength.Ni coating was beneficial for the improvement of the oxidation resistance of the surface of Cu base material.The Ag coating was helpful for the bonding of the Ag filler layer.Low-temperature sintering joining of bare Cu was conducted using Ag-Cu nanoparticle composite paste with the mass ratio of 2:1 for Ag:Cu,and the shear strength of the joint at room temperature was 52 MPa.The Ag and Cu nanoparticles with excellent sintering activities contributed to the metallurgical bonding with the base materials.Meanwhile,Ag and Cu nanoparticles could develop the nano-scale Ag-Cu solid solution at the interface during sintering,which was beneficial to the sintering.The joint strength increased with the increase of Ag content in the composite filler.This was due to the fact that the bonding of Ag with Cu base material was better than that of Cu with base material.Secondly,Ag-Ag sintering in the composite paste was easier than Ag-Cu sintering.In addition,the large particle size of the Cu nanoparticles in the composite paste gave rise to the formation of large voids around them.Thus,the addition of Ag nanoparticles with small particle size contributed to the reduction of the porosity around the Cu nanoparticles and the improvement of the mechanical properties of the joint.